This invention relates to the art of forming an oriented hollow article of a moldable organic plastic material wherein the plastic material is susceptible to improvement in properties by orientation. A principal object of the present invention is to obtain a rapid operating cycle despite the fact that the particular plastic may require prolonged residence in an injection station which would result in a prolonged operating cycle. It is also an objective of the present invention to obtain freedom from opacity in the final article caused by crystalline regions.
The art teaches various methods for obtaining blow molded articles of organic plastic material from a pressure molded parison, such as in U.S. Pat. No. 3,349,155 and Re. 27,104. Generally these methods are characterized by forming a parison in a parison mold on a core, placing said formed parison into a blow mold and expanding said parison in the blow mold by means of fluid pressure.
While the blow molding operation tends to impart orientation to the article, the degree and type of such orientation is difficult to control and therefore it is difficult to obtain the advantageous properties in the article that multi-axial orientation is capable of providing. These advantageous properties are provided when orientation is carried out under the most desirable conditions of temperature distribution by stretching and circumferentially expanding said parison.
It is known that the control of orientation depends largely upon the control of the temperature of the parison just prior to orientation. It is found that such temperature control is best obtained by enclosing the parison prior to the orienting step in an environment, such as a mold whose surfaces may provide conductive or radiant heat exchange with the corresponding surfaces of the parison.
Plastic articles exhibiting the properties of multi-axial orientation, e.g., bottles and other hollow containers, are frequently made in two stages. The first of these two stages is the production of the parison usually by injection molding, or by other pressure molding procedures such as comparison molding and extrusion. The first stage thus serves to convert the chosen plastic into a parison of predetermined shape, having a wall thickness distribution suitable for the production of the final article shape and usually also having a neck or rim configuration suitable to receive a closure appropriate for the said article. If injection molded, the parison is normally removed from the injection mold typically consisting of a mold cavity, a core and a neck (or rim) mold, by extracting it from the mold cavity, by extracting the core therefrom and, either before or after its transformation into the finished article, by releasing it from the neck mold. All of these steps and procedures are well known in the injection molding art. The parison so produced is relatively cold, having a temperature substantially below that required for successful orientation, the reason being that the said parison could otherwise not be extracted from the mold cavity nor separated from the core without significant damage to its dimensions. If the outer surface of the parison is at an elevated temperature, it tends to adhere to the mold cavity and deform in the course of extraction. If the inner surface is not cold enough, it will adhere to the core. Moreover, when extracting the core, vacuum is created in the space within the parison from which the core is extracted and therefore the parison must be strong enough not to deform under the influence of the ambient atmospheric pressure acting upon the outside thereof. In order for the parison to attain the requisite strength, it must be cooled well below that temperature at which it is desirable to form it into the finished article.
The parison so made is next subjected to a second stage of operations wherein it is heated to that temperature at which its transformation into the finished article is to take place and, once at that temperature, it is expanded under conditions imparting the desired orientation.
Such two-stage operations are well known in the art. For example, in a machine designated as RHB 5 built by the Cincinnati-Milacron Company, conventional parisons that have been previously injection molded in a conventional manner are passed through parallel banks of infrared heaters, usually while being rotated around their axes to insure improved heating and, upon reaching the desired temperature, placed into a blow mold in which a stretch mandrel extends the length of the parison to a predetermined degree, followed by expanding the extended parison into conformance with the blow mold by means of a pressure fluid. Other similarly acting devices are well known and described, e.g., in the February and March, 1976 issues of Modern Plastics (a McGraw-Hill Publication). One such device is known as the Model 650 Machine built by Nissei Plastics Industrial Co., Ltd. of Japan, a schematic description of which is given in U.S. Pat. No. 3,944,643 and U.S. Pat. No. 4,105,391. In that machine, the parison is injection molded and cooled in the injection mold which includes a core, to a temperature at which it is easily removed from said core (according to claim 1 of the above U.S. Pat. No. 3,944,643). The parison is then transferred by means of a neck mold to a heating environment designated as a reheating mold (claim 1) or heating device (Column 1, line 6), in which its temperature is raised to that desired for orientation, by means of external and, if found necessary, also internal heaters which may be the radiant or contacting type. Once the parison reaches the desired temperature, it is transferred to a blow mold in which it is converted into the finished article by stretching and blowing, as above described in connection with the RHB 5 Machine.
The disadvantages of such two-stage operations include a slow cycle due to the fact that the parison must first be cooled from the temperature at which it is molded to near room temperature, only to be heated once more to the relatively high orientation temperature, with a corresponding waste of heat. In addition, heating of the parison is unsatisfactory because, it being made of an organic polymer and hence a poor heat conductor, uniform heating across its wall thickness is very difficult and time consuming to obtain.
In addition to the above two-stage procedures, a single stage operation may be employed, wherein the parison is cooled after having been molded only enough to lower its average temperature substantially to that chosen for orientation and the transformation of the parison into the finished article is carried out, for example, by stretching and blowing, after it has attained a uniform cross-sectional temperature distribution corresponding to the above average temperature, preferably in a tempering mold. Such single stage operation is described in my previous U.S. Pat. No. 3,966,378 according to which the parison is injection or compression molded, cooled to a predetermined, limited degree in the injection or compression mold, transferred by means of the injection core into one or more tempering molds for equalization of its temperature distribution and then transferred into a blow mold for finishing of the final article under conditions of biaxial orientation. Such single stage operation lacks the disadvantages of excessive operating cycle, waste of heat and non-uniform temperature distribution at the time of orientation and it can provide the basis for significant improvement in the operating cycle.
A further improvement is shown in my previous U.S. Pat. No. 4,151,248 according to which the parison is provided in a formable condition at a temperature substantially above that required for orientation of said plastic on a temperature controlled first core in a temperature controlled first mold, for example, an injection mold, with the first core and first mold being held at temperatures substantially below that required for orientation of the plastic. The heat content of the parison is rapidly altered by means of heat exchange with the first core and first mold to result substantially in the parison having an average temperature suited for orientation and with an unequal distribution of temperature resulting across the walls of the parison. The resulting parison is then transferred to a closed environment, for example a tempering mold, for equalizing the temperature distribution within the walls of the parison and to attain a temperature throughout corresponding to the chosen orientation temperature of the plastic.
While this procedure represents a significant improvement, thick parisons still result in excessive time delays for cooling in the injection mold and in the tempering mold.
My copending U.S. Pat. application Ser. No. 120,266, now U.S. Pat. No. 4,352,777 responds to this problem by providing more than one tempering mold, with the tempering molds in spaced relationship to each other, for retaining and tempering parisons prior to orientation and blowing in a prolonged tempering cycle. However, delays are still attendant upon retention of the parison in the injection mold.
An additional problem in the art is to avoid undesirable opacity in the final article caused by crystalline phases. These materials are amorphous while molten and while in a glass phase and crystallize as they cool from the molten state. The crystalline material usually has a different index of light refraction than the amorphous phase. The size and degree of dispersion of the crystallites within the amorphous phase depends on the amount and rate of cooling from the molten state; if too short and too slow, the resulting composite of amorphous and crystalline phases may exhibit sufficient opacity to be undesirable in a food and beverage container. Attempts to expedite the processing cycle by unduly rapid removal of the parison from the injection mold may result in such undesirable opacity.
It is, therefore, a principal objective of the present invention is devise a method which enables an improved operating cycle.
It is a particular object of the present invention to provide such a method which does not interfere with the normal operation of the method and apparatus.
It is a still further object of the present invention to provide a method as aforesaid which is capable of achieving the aforesaid advantages without the necessity of constructing an extremely bulky and inefficient apparatus.
It is a further object of the present invention to devise a method as aforesaid which obtains a final article without undesirable opacity.
Further objects and advantages of the present invention will appear hereinbelow.